1. Field of the Invention
The present invention relates to an optical pulse generating apparatus, and more particularly, to an optical pulse generating apparatus having a semiconductor laser medium as its light source.
2. Description of Background Information
There are optical pulse generating apparatuses which have a semiconductor laser medium as a light source, and are configured in such a way that an optical fiber for forming an optical waveguide path is coupled to the light-emitting face of the semiconductor laser medium, forming an external resonator, and mode locking oscillation of the semiconductor laser medium is produced to generate optical pulses.
With such an optical pulse generating apparatus, generation of optical pulses with a narrow spectrum width of 100 GHz or below and a pulse width as narrow as 50 pS (picoseconds) or below is desired. For the generation of such optical pulses, it is necessary, due to lower wavelength-dependency of the reflectivity by a uni-mode fiber, to reduce the reflectivity R.sub.2, of the non-reflecting coating on the end face of the semiconductor laser medium where the optical fiber is coupled, to a level about R.sub.2 .apprxeq.2.times.10.sup.-4. This value is however impractical, and actually it is difficult to ensure the reproducibility at R.sub.2 .apprxeq.10.sup.-2 or below. As the reflectivity R.sub.2 increases, the longitudinal mode, which is dependent on the element's length, of the semiconductor laser medium (this mode will be hereinafter called "device mode") is enhanced, to produce an oscillation spectrum as indicated by the solid line in FIG. 1. This increase in R.sub.2 reduces the level of the main oscillation mode component, thus imposing restriction on narrowing the spectrum width and pulse width of optical pulses.